A microfluid circuit is known from the patent application FR-A-2 873 171 of the applicants, which is provided in an appropriate material such as for example PDMS (poly-dimethylsiloxane) having microchannels having typically a width of about 100 μm and a depth of 50 μm, in which very small flow rates of fluids such as air, water, oil, reagents, etc, can flow. A laser beam, the wavelength of which is not absorbed by the constitutive material of the circuit, is focused on the interface of a first fluid flowing in a microchannel and of a second fluid at least locally present in this microchannel, to force or to stop the flow of the first fluid in the microchannel, to fractionate it in drops, to mix it with the second fluid, etc., the focusing of the laser beam on the fluid interface generating a temperature gradient along this interface and causing a capillary heat convection movement of fluids.